Tuesday 26 August 2008 19.01 EDT
First published on Tuesday 26 August 2008 19.01 EDT

Air accident investigators were last night trying to pinpoint why a Ryanair flight carrying 168 passengers lost cabin pressure over France, forcing an emergency descent that left British holidaymakers believing they were going to die.

Experts are examining the 737-800 jet at Limoges international airport after an incident that had passengers "whimpering" in fear, according to one witness.

Arctic explorer Pen Hadow, a passenger on board the Ryanair flight from Bristol to Barcelona Girona airport, said passengers cried with relief as the plane landed safely on Monday night after plunging from more than 30,000ft to 8,000ft in a few minutes.

"I would say some people thought we were going to die," said Hadow, who was travelling with his wife and two children. His nine-year-old son was one of 16 people taken to hospital suffering from ear problems after pilots brought the plane down rapidly to an altitude where passengers could breathe. All were later discharged.

Hadow described hearing a "loud and unnerving sound" as the cabin pressure dropped. "I knew it wasn't an engine problem. There was a sudden drop in temperature and a rush of cold air. You think to yourself 'God, is there a hole in the aircraft?' It actually felt like someone had opened a door at the back of the aircraft. It was incredibly cold. The next thing, the oxygen masks were dropping."

Charlotte Thorthon, a medical student who was on the plane, said: "I sent my mum a text: 'I love you and I am going down'." Describing a sudden "whooshing sound" as the cabin pressure dropped, the 22-year-old said no announcement was made to tell passengers what to do or what was happening. "For about half an hour no one told us anything, we didn't know what was happening at all, we were quite panicky," she said. "After half an hour we were told to take the oxygen mask off, and then the plane landed and we were fine."

Ryanair rejected a claim by Hadow that some masks did not deliver enough oxygen. The airline's chief executive, Michael O'Leary, said Ryanair engineers had inspected the craft overnight and confirmed the masks were working properly. He also defended the lack of announcements from the crew. "We have to require that the pilots and the cabin crew deploy their oxygen masks and they can't be making PA announcements while they have their oxygen masks on," he told the BBC.

The French civil aviation authority said initial inspections revealed no obvious problems with the plane.

Air, low in oxygen at cruising altitudes, is sucked into the cabin through the engines and, after it is mixed with filtered and recirculated cabin air, is controlled by an "outflow valve" that regulates its flow through the aircraft. If that supply is suddenly disrupted by a malfunctioning valve or hole in the cabin, pilots have a short time to bring the plane down to an altitude where the air is breathable - around 8,000ft. It was this rapid, controlled descent in response to a warning signal in the cockpit that led some passengers on Flight FR9336 to believe that the aircraft was plunging to the ground.

"One thing that can happen is that the outflow valve can fail and become open, so all the air rushes out," said David Learmount, operations and safety editor of Flight International magazine.

Wear and tear, which can create cracks in an aircraft fuselage through which air escapes, was another possible cause. "A hole can appear because some damage was done which nobody notices and, because of the constant stress of pressurising and depressurising the cabin, it becomes bigger over time," he added.

Dublin-based Ryanair said the aircraft was five years old and was last serviced a month ago. The flight captain had flown with Ryanair for five years and seven months and had more than 13,400 hours flying experience.

Concerns have been raised over pilot fatigue at Ryanair but experts say the airline has addressed them. O'Leary admitted to the Guardian last year that some pilots had flouted guidelines and made dangerous landing approaches. "What we had in these cases was jet jockeys deciding 'I am better than Ryanair SOPs [standard operating procedures].' We don't want anybody doing that," he said.

Ryanair's disaster contingency operation for serious incidents has been described as "very impressive" by aviation professionals who have visited it.

Learmount also defended Ryanair's maintenance record. "It has the youngest fleet of aircraft in Europe and their maintenance organisation is first class," he said.

In July, in the most dramatic decompression incident of recent times, a Boeing 747-400 carrying 365 passengers and crew operated by Australia's Qantas was forced to descend rapidly after an oxygen tank exploded en route to Melbourne, blowing a hole in the fuselage.